It is known in the oil industry to use a rotating sucker rod string to operate a downhole pump known as a progressive cavity pump. Such a pump is commonly used in primary production from an oil reservoir or possibly to pump water from a gas well, etc. These pumps are useful because of their ability to pump viscous fluids and fluids containing significant amounts of solids. The rod string is generally located within a well tubing of circular cross section lining the well shaft of a well and the string is rotated by a geared mechanism atop the well. Crude oil passes upwardly through the gap between the string and tubing to ground level during well operation.
Torsional forces are loaded onto the string during well operation and this arrangement can lead to a warping of the string such that portions of the rotating string rub directly against the well tubing. Typically string deformation is such that a rod rubs against a localized area of the tubing rather than the entire inner circumference of the tubing. Such rubbing thus leads to local wear of the tubing and rod and is generally undesirable. It is also possible for such rubbing to arise due to a crooked tubing or tubing deliberately deviated during installation.
An approach to combatting this wearing problem has been the use of centralizers located on the string at axially spaced locations. There is known in the oil industry a centralizer having a hollow circular center. It has an outer diameter having a cross section exceeding that of the rod on which it is mounted but which is smaller than the interior diameter of the tubing within which it is located. The outer diametrical cross section defines arc portions which lie on a common circle centered on a central axis of the hollow center. As a string becomes deformed or if the well tubing is crooked, for example, one or more of the centralizers will come into contact with the tubing and direct contact between the string and tubing is prevented. The arrangement is such that, as the centralizer comes into contact with the tubing, the centralizer rotates with the string and thus rubs against the stationary tubing. The rubbing of the rotating centralizer against the stationary tubing damages the tubing reducing its life.
The use of a centralizer mounted to be capable of rotation with respect to the rod, and which does not rotate with the rod when the centralizer is in contact with the well tubing, may reduce tubing wear and thereby extend tubing lifetime.